can't start beginBackgroundTask swift 3

Question

Sorry I am stuck, but I am trying to start background task (XCode8, swift 3)

Example from here: https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html#//apple_ref/doc/uid/TP40007072-CH4-SW3

In AppDelegate.swift:

func applicationDidEnterBackground(_ application: UIApplication) {
    var bgTask: UIBackgroundTaskIdentifier = 0;
    bgTask = application.beginBackgroundTask(withName:"MyBackgroundTask", expirationHandler: {() -> Void in
        print("The task has started")
        application.endBackgroundTask(bgTask)
        bgTask = UIBackgroundTaskInvalid
    })
}

The app has never shown "The task has started" message. What am I doing wrong?

Answer 1

The expiration handler block gets called after a period of time in background (usually 5 minutes or so). This is meant to be used to write the clean up logic if in case you background task is taking a lot of time to complete.

There is nothing wrong with your code, you just need to wait in the background for the background task to expire.

Answer 2

Your use of the background task is all wrong. It should be something like this:

func applicationDidEnterBackground(_ application: UIApplication) {
    var finished = false
    var bgTask: UIBackgroundTaskIdentifier = 0;
    bgTask = application.beginBackgroundTask(withName:"MyBackgroundTask", expirationHandler: {() -> Void in
        // Time is up.
        if bgTask != UIBackgroundTaskInvalid {
            // Do something to stop our background task or the app will be killed
            finished = true
        }
    })

    // Perform your background task here
    print("The task has started")
    while !finished {
        print("Not finished")
        // when done, set finished to true
        // If that doesn't happen in time, the expiration handler will do it for us
    }

    // Indicate that it is complete
    application.endBackgroundTask(bgTask)
    bgTask = UIBackgroundTaskInvalid
}

Also note that you should use beginBackgroundTask/endBackgroundTask around any code in any class that you want to keep running for a short period time even if the app goes into the background.